The study was conducted with 10 people in Tokyo and 10 people in Fukushima prefecture, all of whom were asked to wear polyester-and-paper face masks for approximately 60 hours per week, and then send the masks back to the center in plastic bags. So far, the researchers have analyzed masks only from one week: the week of Feb. 19-25.

Eight of the 10 masks from Fukushima contained cesium-137, a radioactive form of the element cesium that’s commonly released after nuclear accidents. Five of those eight also tested positive for traces of cesium-134 another radioactive cesium isotope.

On average, the study found the masks from Fukushima contained radioactive cesium in the amount of 4.3 becquerels (Bq) –- a common way of measuring radioactive emissions from a material. That was comprised of 1.73 Bq of cesium-134 and 2.52 Bq of cesium-137.

If a person inhaled this amount of radioactive cesium, they would get approximately 0.082 microsieverts of radiation exposure per week, explained Shogo Higaki, the researcher from the Radioisotope Center. That would be a tiny addition to what people are normally exposed to anyway, he said. For comparison, a person in Tokyo is now normally exposed to about 0.06 microsieverts per hour of radiation, Mr. Higaki said. That works out to around 10 microsieverts per week.

Meanwhile, only one of the masks from Tokyo tested positive for radioactive cesium, and the amount of radiation was around 14% of that found in the masks from Fukushima, at 0.22 Bq of cesium-134 and 0.38 Bq of cesium-137, for a total of 0.6 Bq of cesium. Mr. Higaki also noted that environmental levels of radiation found in the Japanese capital had dropped to levels close to those before the nuclear accident occurred.

That admittedly sounds very low. But Mr. Higaki added, “There are various ways to interpret the data.”

To address the focus of the study, tests measured both the amount of pollen and the amount of cesium found on the masks and the bags. Further tests then measured how much radioactivity could be attributed to the pollen on the masks. “Based on the results so far, pollen does not seem to be a main source of cesium,” announced Mr. Higaki. He also noted that radioactive contamination is more likely from sources like dust and dirt than from airborne particles.

To be sure, the week in February during which the measurements were taken is at the very start of pollen season, so a lot can change yet.

Masks will be tested at the Radioisotope Center through April 14 from the 20 study participants, Mr. Higaki said.

About Japan Real Time

Japan Real Time is a newsy, concise guide to what works, what doesn’t and why in the one-time poster child for Asian development, as it struggles to keep pace with faster-growing neighbors while competing with Europe for Michelin-rated restaurants. Drawing on the expertise of The Wall Street Journal and Dow Jones Newswires, the site provides an inside track on business, politics and lifestyle in Japan as it comes to terms with being overtaken by China as the world’s second-biggest economy. You can contact the editors at japanrealtime@wsj.com